Currents in narrow pores

ABSTRACT

Current fluctuations typical of those observed with biological channels which are ion selective and inhibited by divalent cations and protons have been observed in synthetic membranes, and a process for controlling permeability of a synthetic membrane to ions and uncharged molecules may be used in a switch or sensor, for example to monitor the content of a solution, to trigger macroscopic events by local microscopic changes and to alter the ionic content of a solution e.g. desalination.

The present invention relates to a process for controlling thepermeability to ions or uncharged molecules of a synthetic membrane, andto switching and sensor devices which use this effect.

The characteristic feature of ion channels in biological membranes isthat when an electric potential is applied across the membrane, theresultant current fluctuates between a conducting and a non-conductingstate 1!. To date most explanations of this phenomenon have been basedon the notion that conducting and non-conducting states represent "open"and "closed" configurations of the channel, and that the transitionbetween them results from the physical movement of part of the channelstructure,--the opening and closing of some kind of "gate" 2!.

The applicants have found that current fluctuations typical of thoseobserved with biological channels can be observed with purely syntheticmembrane filters that may be devoid of any added lipid or protein. Twoother features of biological ion channels are ion selectivity andinhibition by divalent cations and protons 2!; these too are displayedwhen current across synthetic membrane filters is measured. It isbelieved that the results may be interpreted in terms of the specialproperties of ion conductance through water that is confined near aninterface; they may be observed whenever the contribution of bulkconductance is minimal, such as in a narrow pore having a high surfaceto volume ratio.

Flow of electrolytes or non-electrolytes through pores in biologicalmembranes that are induced by agents as varied as certain haemolyticviruses, bacterial or animal toxins, immune molecules or detergents atsub-lytic concentration show properties that are remarkably similar fromagent to agent. These include inhibition by divalent cations, withrelative efficacy Zn²⁺ >Ca²⁺ >Mg²⁺ and, when the agents are incorporatedinto planar lipid bilayers with an applied voltage across them,fluctuations in current typical of "single channel" openings andclosings of endogenous ion channels; in several of these systems protonsalso inhibit flow (with relative efficacy H⁺ >Zn²⁺).

The applicants have studied flow (i.e. current) of non-electrolytes andelectrolytes through purely synthetic membrane filters and find the sameeffect of divalent cations and protons, provided the membrane pores arenarrow enough. It is concluded that these effects are features of flowalong any liquid--solid interface, that can be observed wherever thecontribution of bulk is minimised (as in the case of very narrow pores).

The word "pore" in the context of the present invention is intended toinclude a hole through a membrane or the space between two apposedsurfaces that may be two solids or two lipid monolayers.

For simplicity the term "membrane" is used in the present description,but it is to be understood that, unless otherwise specified, this termencompasses films or sheets of any shape and thickness and whosethickness may or may not be uniform, and also encompasses thecombination of two solids or two lipid monolayers whose relativeconfiguration forms at least one pore as defined above.

The possible applications of a switching device based on syntheticorganic materials are wide ranging. Some are listed below:

(1) a switched pore can control the transfer of particular solvent andsolute molecules across an otherwise impermeable membrane which hasapplications in chemical engineering.

(2) a pore that may be switched from low to high conductance can alsoserve as a sensitive detector of particular conditions such as pH andionic strength or even an elevation in the concentration of a particularsolute molecule. Such a detector may have a very high gain (oramplification) in that the concentration change required to switch thepore conductance may be very much smaller than the detected flow throughthe pore when it is opened. Applications of this effect include chemicaland medical monitoring of solutions with the rapid detection of changesin specific constituents of the solution.

(3) A pore is switched from high to low conductance by the particularconditions that prevail within the pore or in the immediate vicinity ofits two ends. Thus by special preparation of this very localenvironment, for example by introducing specific chemical entities inthese regions, the macroscopic bulk flow through the pore is controlledby changes in the chemical environment of these microscopic regions.This again renders it possible for macroscopic events to be triggered byextremely local changes which will have applications in process control.

(4) As in semiconductor physics, once a switch has been developed, thenamplifying devices with controlled gain are obtained by the applicationof negative feedback around the switch and oscillators are obtained bythe application of positive feedback around the switch. The rapidity ofthe switch leads to the possibility of high frequency devices.

(5) Since flow of ions is more sensitive to inhibition by divalentcations and protons than is flow of water, the membrane may be used toalter the ionic content of a solution; an important application in thisrespect is in desalination processes.

The present invention provides a process for controlling permeability toions and uncharged molecules through a pore as defined herein havingdiscrete states of differing permeability and being of less than 10nanometers radius or 20 nanometers width, which process comprisesplacing the pore between a first solution containing an ion (thetransport ion), or uncharged molecule capable of passing through thepore, and a second solution;

and controlling the permeability of ions or uncharged molecules throughthe pore by means of the transport ion concentration in the firstsolution and/or the proton or multivalent ion concentration in the firstand/or second solution.

Thus the invention also includes a process in which the laminar flow ofions or uncharged molecules between two apposed membranes, separated by<20 nm, may be controlled by the proton or multivalent ion concentrationas described above.

The present invention also provides a switch for ionic currentcomprising a pore of less than 10 nm radius through a synthetic membranefilm or comprising two apposed synthetic membranes less than 20 nm apartforming a pore between the membranes, said pore being simultaneously incontact with a first ionic solution containing an ion capable of beingtransported through the membrane pore, the transport ion, and with asecond ionic solution,

such that when the switch is placed in an electric circuit, flow ofionic current though the pore is controlled by transport ionconcentration in the first ionic solution and/or the proton ormultivalent ion concentration in one or both of the said ionicsolutions.

The present invention also provides a sensor for sensing a parameter ina solution such as pH, ionic strength or solute concentration, saidsensor comprising a pore of less than 10 nanometers radius through asynthetic membrane or comprising two apposed synthetic membranes lessthan 20 nm apart forming a pore between the membranes,

such that when the pore is placed in contact with the solution whoseparameter is to be detected, the ionic current passing through the poreis indicative of the presence, absence or level of the parameter to bedetected.

DETAILED DESCRIPTION

The term "ionic solution" in this context refers to a solutioncontaining ions which may be e.g. a solution of an ionic material, suchas a salt, dissolved in an appropriate solvent, or may be simply aliquid, such as water, in which a proportion of the liquid exists in theform of ions.

Suitable ions which may be transported through the membrane pore includealkali metal ions (e.g. Li⁺, Na⁺, K⁺, Rb⁺), halide ions (e.g. F⁻, Cl⁻,Br⁻, I⁻) and multivalent ions (e.g. Mg²⁺, Ca²⁺, SO₄ ²⁻, phosphate).Preferred ions will depend on the use to which the process is beingapplied e.g. desalination.

Suitable solute molecules which may be transported include water solublemolecules whose effective size is less than the effectivecross-sectional area or width of the pore (e.g. sugars such as glucose,amino acids, certain hormones, neurotransmitters, drugs, immunemolecules or polymers such as polyethylene glycol).

Current switching behaviour typical of that observed in an ionic channelin a naturally occurring membrane may be reproduced by a syntheticmembrane containing narrow pores. The membrane may be a plastic filmsuch as a polycarbonate or other polyester, e.g. polyethyleneterephthalate (PETP). The applicants have investigated PETP films of 5or 10 micrometers thickness and found them to display the effect of theinvention, but thinner or thicker films could be used, depending on therequirements of the use to which the process is being applied. Forexample, the minimum thickness may depend inter alia on physicalcharacteristics such as the ability of the film to be handled by themanufacturer or user and on the pressure the film is subject to duringuse. For electrical devices (e.g. sensors) thin films are likely toprove optimal. For flow devices (e.g. desalination) thick films arelikely to prove optimal; alternatively laminar flow between two apposedmembranes may be used.

The applicants believe that other synthetic membranes exposed to thetrack-etch procedure described below are also suitable for use in thepresent process; these include polymers that are positively charged aswell as those like PETP that are negatively charged after track etching.In the case of positively charged membranes, some of the properties e.g.selectivity and inhibition by protons can be reversed.

Narrow pores in plastic films such as polyester, e.g. PETP, may beformed by irradiating the film with heavy ions such as ¹³² Xe, ¹²⁹ Xe,⁸⁴ Kr or ⁵⁹ Co accelerated in a cyclotron, to produce tracks; thepolymer is then treated with a hot alkali 3! to hydrolyse parts of thepolymer thereby causing local solubilization of the film materialparticularly in the regions affected by the heavy ion irradiation. Thewhole process is called track-etching.

In the case of laminar flow between two apposed membranes track-etchingof the membrane in order to create narrow pores is unnecessary, butchemical treatment e.g. etching alone of the surfaces may be required.

In the track-etching process cylindrical or conical pores are formedwhich penetrate the film. The pore diameter depends on the energy andspecies of heavy ions used for the film irradiation, on the type ofpolymer used and especially on the conditions of the etching process.

To achieve membranes which may display the effect of the presentinvention, beams of heavy ions may be used which typically have thefollowing energies: ¹³² Xe(121 MeV), ¹²⁹ Xe(124 MeV), ⁸⁴ Kr(74 MeV) and⁵⁹ Co(68 MeV), with an ion fluence of from 6×10⁵ to 1×10¹¹ cm⁻².Appropriate alkalis and temperatures for the use in this method would beevident to the ordinarily skilled person, and for materials such as PETPinclude e.g. 0.1M NaOH, 1.0M K₂ CO₃ and 2.5M K₂ CO₃ used at atemperature of about 80° C.

For the purposes of the present invention the pore diameter or widthshould be less than 20 nanometers.

The chemical etching of pores into a material such as polyethyleneterephthalate results in hydrolysis of the PETP generating free carboxylgroups making the pores in the membrane more hydrophilic, but otherpossible membrane materials could be used.

The density of pores in the membranes that the applicants have studiedso far range from 1 to 5×10⁹ pores per cm² but there is no reason tosuppose that higher pore densities could not also be used. Theproperties of individual pores are independent of pore density. Forelectrical devices (e.g. sensors) the pore density should be low. Forflow devices (e.g. desalination) the pore density should be high; analternative mode in the latter case is for flow to be between twoclosely apposed membranes such that the space between them is less than20 nm.

Transfer of ions, i.e. ion current across such membranes may exhibitrapid switching from a high conductivity to a low conductivity state,even when the voltage across the membrane is held fixed. The rapidswitching shown by such a narrow pore membrane resembles the rapidswitching shown by biological ion channels. This switching may beobserved whenever the ratio of surface area to volume of the conductingphase is high, such as in narrow pores or in the space between twoclosely apposed solid surfaces or lipid monolayers. Switching can bemodified and controlled by controlling the amount of particular ionssuch as protons and multivalent ions in solution. This control may beachieved by adding protons or a source of multivalent ions, and may bereversed by adding alkali to remove such protons or a sequestrant suchas ethylene diamine tetraacetic acid (EDTA) to remove multivalent ions.The control of conduction through the narrow pore includes control ofthe transfer of water and uncharged molecules as well as ions.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A: shows how current varies as a function of applied potential fortwo filters.

FIG. 1B: shows how conductivity varies in the presence of a solution ofpolyethylene glycol of varying molecular weight.

FIG. 2: shows the dependence of pore conductance on ionic strength as adouble logarithmic plot.

FIG. 3A: shows representative current fluctuations across filters as afunction of time.

FIG. 3B: shows the amplitude distribution histograms of the current inthe predominant conducting states.

FIG. 3C: shows time distribution histograms for the high and lowconducting states.

FIGS. 4A and 4B: shows how the conductivity varies with theconcentration of dilvent cation or proton.

FIG. 4C: shows how the logarithm of the ratio of the time constants forthe high and low conducting states varies with the concentration ofdivalent cation or proton.

FIG. 5: shows the apparatus used to monitor ion currents through one ora few pores in a track-etched polyethylene terephthalate (PETP) filtercontaining many pores.

FIGS. 6A and B: shows the frequency distribution of ionic conductivity(A) and calculated diameter (B) of pores in a track-etched PETP filter.

FIG. 7: shows patch--pipette currents from a track-etched PETP filter.

FIG. 8: shows ionic selectivity of pores in track-etched, PETP filtersand in such filters coated with PMVP.

The following examples illustrate the invention.

EXAMPLE 1

The filters used were approximately 5 μm thick and made of polyethyleneterephthalate (PETP; "Lavsan"). After heavy ion bombardment to inducenuclear tracks, the filters were "etched" with hot alkali; thistreatment breaks (hydrolyses) some of the ester bonds generating freecarboxyl groups. These may be negatively charged and make the filtersmore hydrophilic 3!. Two separate filters, having slightly differentapparent pore sizes were used. They are referred to throughout as filter1 and filter 2. The pore density for each filter was approximately 1pore/cm².

Filters were clamped between two small chambers (volume 0.3 ml each)which contained KCl (0.1-3M), buffered with 0.005M Hepes pH 7.4,respectively. Current was measured at room temperature with Ag/AgClelectrodes monitored with a virtual grounded operational amplifier witha feed-back resistor of 10⁹ ohms.

The membrane switches between a high conducting or "on" state and a lowconducting or "off" state. The reversal potential, i.e. the sign andmagnitude of the voltage (ψ) at zero current was used to deduce theselectivity of the filters for cations over anions, in the presence of afive-fold gradient of KCl. The modal current in the high conductingstate is obtained from amplitude histograms as described hereafter. InFIG. 1A this current is plotted as a function of applied potential forfilter 1(□) and filter 2(). The selectivity (t₊) was calculated fromthe reversal potential (ψ) using the equation (I): ##EQU1##

The terms "trans" and "cis" are used to identify the compartments oneither side of the membrane.

The experiments illustrated in FIG. 1A show the reversal potential to be35 mV for each filter, indicating a selectivity value for cations overanions (t₊) of 0.93.

The relative pore sizes of the two filters are estimated by assessingthe effect on conductivity of solutions of a non-electrolyte,polyethylene glycol (PEG), of varying molecular weight. If thenon-electrolyte is able to pass through the pore, conductivity isdecreased; if not, it is unaffected. By determining the size of thenon-electrolyte that is unable to pass through the filter, an estimateof its average pore size can be obtained.

The conductivity of the pore in the presence or absence of 20% (v/v)polyethylene glycol (PEG) of differing molecular weights is compared.The conductances (G) are calculated from the modal current of thehighest conducting state at a potential of 0.2V for each filter in thepresence of 20% PEG relative to that in the absence of PEG. Theconductances for filter 1(□) and filter 2() are shown in FIG. 1B andexpressed as set out in equation II as the ratio (A) of that parameterwith the same parameter but measured in the absence of the filter:##EQU2##

The approximate size of PEG which results in "cut off" is considered tobe the size which yields a value of A of 0.5. This size was found atmolecular weights of ˜3000 for filter 1 and ˜1000 for filter 2. Based onthe "exclusion" volume of PEGs in water, these correspond to pore radiiof about 1.4 nm for filter 1 and 1 nm for filter 2. It is seen that thepore in filter 2 appears to be somewhat narrower than that in filter 1,compatible with a lower conductance (FIG. 1A).

The dependence of pore conductance on ionic strength is shown in FIG. 2as a double logarithmic plot. Identical solutions containing KCl at theconcentration indicated in FIG. 2 with 10⁻⁴ -3×10⁻³ M EDTA and 0.005MHepes at pH 11(♦), 8.3(), 7.5(▴) or 6.5(▪) bathed each face of filter2. Conductance (G) of the highest conducting state at an appliedpotential of 0.8V is shown. Similar data were obtained at an appliedpotential of 0.2V, except that at pH 6.5 the high conducting state wasnot observed.

At pH 11 conductance is almost independent of ionic strength for current(I)≦0.1. At lower pH values the conductance has a linear dependence on Ibut with a slope much less than unity. These results suggest thatnegative charges either in the pore or close to its mouth exertconsiderable influence on the conductance by causing the local cationconcentration greatly to exceed that in the bulk solution. The change ofslope over the range of pH values tested suggests that the negativecharges titrate with an effective pK around neutrality. Currentfluctuations are maximal around this pH, with smaller, fewerfluctuations above or below it 4!. Such an effect has recently beenobserved with S. aureus α-toxin induced pores across lipid bilayers also5!.

Current fluctuations across the filters were measured and typicalrecords of current are shown in FIG. 3A. Filter 1 was bathed in 0.1MKCl, 0.005M Hepes pH 7.4 without (i) or with 3.10⁻⁴ M (ii) or 3.10⁻³ MCaCl₂ (iii); filter 2 (iv) was bathed in a similar medium without CaCl₂at pH 8.0; currents at 0.2V were recorded on videotape using a BiologicPCM Instrumentation Recorder and subsequently analysed using CambridgeElectronic Design Patch and Voltage-Clamp software. FIG. 3A showsrepresentative traces. Discrete changes between conducting andnon-conducting states are seen.

Analysis of the two predominant states (FIG. 3B) shows the higher one tobe 11 pA and the lower one 2.5 pA for filter 1. For filter 2 the valuesare 3.3 pA and 1.2 pA. The lower state is unlikely to represent a "leak"current, as (a) it changes on the addition of Ca²⁺ (see below), (b) itdisappears altogether at low pH and (c) it is not seen if the filter ismoved so that there is no pore at all between the two chambers.

FIG. 3C presents time distribution histograms for the high and lowconducting states respectively together with the time constants for eachstate obtained by fitting a single exponential to the distribution. Forfilter 1 mean values (±S.D.) of the time constant for the highconducting state (τ_(H)) of 210±59 ms and of the low conducting state(τ_(L)) of 23±8.7 ms (n=8) were found.

The difference between the high and low conductance states varies withvoltage and ionic strength.

The amplitude of the difference between the current of the high and lowconducting states as a function of applied voltage or of the ionicconcentration of the bathing medium are shown in table 1 below; n.d.indicates a value was not determined.

                  TABLE 1                                                         ______________________________________                                        Difference in current between the high and low conducting                     states of filter 2 at different potentials and ionic strengths.                               current/pA                                                    Applied potential/V                                                                             0.1M KCl 0.2M KCl                                           ______________________________________                                        0.2               2.7      4.0                                                0.4               5.5      7.0                                                0.6               8.8      n.d.                                               ______________________________________                                    

Although the absolute value of the higher conductance state dependssomewhat on the pre-treatment of the filter prior to use (e.g. exposureto ethanol, lipids, other chemicals, etc) the difference in amplitudebetween the higher conductance state and the lower one is remarkablyconstant. Other factors, such as pH (FIG. 2) or the presence of divalentcations (see below) do affect both conductance states. On the additionof 0.3 mM Ca²⁺, for example, the difference between the higher and lowerconducting state is decreased to 9.5 and 3.5 pA respectively for filter1 (FIG. 3B). Such intermediate amplitudes also become detectable in theabsence of Ca²⁺ if a long enough recording is analysed. Addition of 3 mMCa²⁺ abolishes the high conducting state completely; high conductance isrestored by the addition of EDTA (not shown). Similar effects areobserved by addition of other divalent cations such as Zn²⁺ or Mg²⁺, orby reduction of pH.

The relative efficacy of divalent cations and protons at reducingcurrent is shown in FIG. 4. Filter 1 was bathed in 0.1M KCl (FIG. 4A) or3M KCl (FIG. 4B) with 0.005M Hepes, initial pH 7.4, H⁺ /OH⁻ (o) ordivalent cations (at pH 7.4: Zn²⁺ (▴), Ca²⁺ (▪), Mg²⁺ (♦)) were added togive the final concentrations indicated. The time-averaged conductance(G) at 0.2V is expressed relative to the maximum observed conductance(G_(o)) at the start of each titration. FIG. 4C presents the logarithmof the ratio of the time constants for the high (τ_(H)) and low (τ_(L))conducting states obtained at 0.2V in 0.1M KCl, 0.005M Hepes at the pHspecified (□--filter 1; ◯--filter 2) or in a similar medium at pH 7.4(filter 1) containing the concentration of CaCl.sub. (▪) or ZnSO₄ (▴)indicated; error bars represent the standard deviation found for filter1 (n=8) at pH 7.4 and for filter 2 (n=4) at pH 8.0.

It is seen from FIGS. 4A and 4B that in both cases H⁺ >Zn²⁺ >Ca²⁺ ≧Mg²⁺with 50% inhibition at molar concentrations of approximately 10⁻⁷.7,10⁻⁴.5, 10⁻³.4, and 10⁻³.0 (0.1M KCl) and 10⁻⁷.5, 10⁻².8, 10⁻¹.7 and10⁻¹.7 (3M KCl) respectively for filter 1, and 10⁻⁸.4, 10⁻⁶.0, 10⁻⁴.2,and 10⁻⁴.2 (0.1M KCl) for filter 2 (not shown). The fact that inhibitionis little different in 0.1M and 3M KCl suggests that simple screening ofsurface charge plays at best a minor role. The data points in FIG. 4Aand 4B are derived from time-averaged conductances. Kinetic analysis (asin FIG. 3C) of fluctuations observed in the presence of divalent cationsand protons is shown in FIG. 4C. Both sets of data show that filter 2 ismore sensitive to inhibition by divalent cations and protons, againcompatible with a smaller apparent pore size (see FIG. 1B).

Ionic conduction in narrow pores may well be similar to that whichoccurs in some hydrated zeolites where it can be shown 6! that only thebare ion moves, although it is at all times hydrated by an essentiallystatic array of water molecules. Calculation 7! shows that the centresof the planar pentagon and puckered hexagon water rings that preservetetrahedral bonding can provide low energy ion binding sites. The originof the rapid switching maybe connected with the fact that lattice sums8! and molecular dynamic simulations 9! show that water in narrow poreswill be electrically ordered. Changes in order could lead to switching8! due to the powerful electric fields generated.

An alternative explanation of the switching is that changes in thenature of the ions absorbed on the pore walls alter the water structurenear the pore walls and hence the pore conductance. In zeolites it isknown 10! that exchange between cations in the bathing solution and thealumina-silicate cage can lead to phase changes in the cages and henceof the water contained within them. Such metastability may account forrapid transitions in the conductance through narrow pores. In abiological system the channel protein must also supply a gatingmechanism sensitive to the membrane voltage 11! and to the presence ofparticular ligands 2!, which may be in series with the water-pore gatingdescribed here. In addition the protein must distinguish betweendifferent cations 12! and carry out other activities such asinactivation 13!.

EXAMPLE 2

Individual pores in track-etched PETP filters containing up to 5×10⁹pores cm⁻² also exhibit selective ion flow and rapid switching betweendiscrete conductance states provided that their overall ionicconductivity is sufficiently low (less than 100 pS in 0.1M KCl).

Such pores can be studied with glass micropipettes (1) having a tipdiameter of about 1 μm using the apparatus illustrated in FIG. 5; ioncurrents through the pores in the filter (between bath and pipette) aremonitored with Ag/AgCl electrodes in the electronic circuit shown. Whenbath and pipette each contain 1M KCl with 0.005M Hepes, pH adjusted to7.4 with KOH the conductance recorded each time the pipette contacts thefilter falls in the range 5 to 2000 pS as shown in FIG. 6 (left handpanel). Assuming that each conductance represents that through acylindrical pore in a 10 μm thick filter, the diameter of that pore canbe calculated and the results of such calculations are shown in FIG. 6,right hand panel. Pore diameters exhibit an approximately normaldistribution with a mean close to 200 Å.

Rapid switching between high and low conducting states is observed whenthe bath and pipette contain 0.1M KCl with 0.005M Hepes, pH adjusted to7.4 with KOH and the overall pore conductance does not exceed 100 pS.Typical results at positive or negative applied potentials are shown forfour different pores in FIG. 7.

Selectivity of ion currents through individual pores using the apparatusshown in FIG. 5 was assessed from the reversal potential (the appliedpotential at which no ion current flows) observed when the pipettecontained 1M and the bath 0.2M KCl each with 0.005M Hepes, pH adjustedto 7.4 with KOH. Cation selectivity, indicated by transference numbers(t₊) in excess of 0.5, is observed for pores in track-etched PETPfilters whereas anion selectivity (t₊ less than 0.5) is observed insimilar filters coated with polyvinylmethylpyridine (PVMP) to conferpositive surface charge on the PETP. In either case pores withconductances greater than 1000 pS show little selectivity (t₊ ≈0.5);selectivity increases, for cations with PETP filters and for anions withPVMP coated filters, progressively as pore conductance diminishes (FIG.8).

REFERENCES

1. Neher E and Sakmann B Nature 260, 799-802 (1976).

2. Hille B. Ionic Channels of Excitable Membranes. Sinauer AssociatesInc., Sunderland, Mass. (1984) pp 277, 303-353.

3. Apel P Y, Didyk A, Kravels L I and Kuznetsov V I Nucl. Tracks Radiat.Meas. 17 191-193 (1990).

4. Pasternak C A, Bashford C L, Korchev Y E, Rostovtseva T K and Lev AA. (1993) Colloids and Surfaces A: Physicochemical Engineering Aspects77 119-124.

5. Bezrukov S. M. and Kasianowicz J J Bull. Am. Phys. Soc. 37, 625(1992).

6. Barrer R M and Rees L V C. Trans Faraday Soc. 56 709-721 (1960).

7. Edmonds D T, Proc. R. Soc. B. 211 51-62 (1980).

8. Edmonds D T. In: Biological Membranes Vol. 5 (Ed D Chapman) AcademicPress Inc. pp 341-379 (1984).

9. MacKay D H C, Berens P H and Wilson K R. Biophys. J. 46, 229-248(1984).

10. Barrer R M and Hinds L. J. Chem. Soc. 1879-1888 (1953).

11. Papazian D M, Timpe L C, Jan Y N and Jan L Y. Nature 349 305-310(1991).

12. Yool A J and Schwarz T J, Nature 349 700-704 (1991).

13. Stuhmer W, Conti F, Suzuki H, Wang X, Noda M, Yahagi N, Kubo H andNuma S. Nature 339 597-603.

We claim:
 1. A process for controlling permeability to ions anduncharged molecules through a pore of a synthetic membrane said porebeing selected from a hole through said membrane and the space betweentwo apposed surfaces, said apposed surfaces forming said syntheticmembrane, said pore having discrete states of differing permeabilityand, when said pore is a hole through said synthetic membrane, said porebeing of less than 10 nanometers radius or, when said pore is formedfrom two apposed surfaces, said pore being of less than 20 nanometerswidth, which process comprises placing the pore between a first solutioncontaining an ion, referred to as the transport ion, or unchargedmolecule capable of passing through the pore, and a second solution;andcontrolling the permeability of ions or uncharged molecules through thepore by means of the transport ion concentration in the first solutionand/or proton or multivalent ion concentration in the first and/orsecond solution.
 2. A process according to claim 1 in which the pore hasbeen created through a synthetic membrane by a track-etch procedure. 3.A process according to claim 1 in which the pore is through a polyestermembrane and is of less than 10 nanometers radius.
 4. A processaccording to claim 1 in which the pore comprises two apposed syntheticmembranes less than 20 nanometers apart.
 5. A process according to claim4 in which the apposed membranes have been etched.
 6. A processaccording to claim 1 where the first solution is salinated water wherebydifferential flow of ions through the pore causes desalination of thewater, the desalination being controlled by means of the concentrationof protons or divalent cations in the first solution.
 7. A switch forionic current comprising a pore of less than 10 nm radius through asynthetic membrane film or comprising two apposed synthetic membranesless than 20 nm apart forming a pore between the membranes, said porebeing capable of exhibiting discrete states of differing permeabilitywhen simultaneously in contact with a first ionic solution containing anion capable of being transported through the pore, the transport ion,and with a second ionic solution,the switch being such that when it isplaced in an electric circuit, flow of ionic current through the poremay be controlled by transport ion concentration in the first ionicsolution and/or porton or multivalent ion concentration in one or bothof the said ionic solutions, causing the pore to exhibit a discretestate of permeability.
 8. A switch as defined in claim 7 in which thepore has been created through a synthetic membrane by a track-etchprocedure.
 9. A sensor for sensing a parameter in a solution, saidsensor comprising a pore of less than 10 nanometers radius through asynthetic membrane or comprising two apposed synthetic membranes lessthan 20 nm apart forming a pore between the membranes, said pore havingdiscrete states of differing permeability,such that when the pore isplaced in contact with the solution whose parameter is to be detected,the ionic current passing through the pore depends on ion and/or protonconcentration in the solution and is indicative of the presence, absenceor level of the parameter to be detected.
 10. A sensor as defined inclaim 9 in which the pore has been created through a synthetic membraneby a track-etch procedure.